1. Field of the Invention
This invention relates to sprocket wheels used with endless tracks used to propel heavy track laying vehicles [i.e., vehicles which use endless tracks rather than tires to contact the terrain over which they are driven, e.g., tractors, tanks, bulldozers, etc.] and, more particularly, to an improved sprocket wheel allowing the endless track to be driven more efficiently at highway speeds.
2. General Discussion
Numerous types of vehicles are frequently used in terrain in which it is difficult for pneumatic tires to operate. Both military vehicles, such as tanks and amphibious vehicles, and civilian vehicles, such as tractors and recreational vehicles, are sometime utilized on terrains which are very soft, for example sand surfaces. Pneumatic tires are not capable of efficient operation on such soft surfaces, as they tend to burrow into the surface, rather than riding across the surface.
Endless track vehicles have been developed for use on terrains in which pneumatic tired vehicles are impractical. See for example U.S. Pat. No. 3,361,488 (Ohrn et al), U.S. Pat. No. 3,688,858 (Jespersen), U.S. Pat. No. 3,734,577 (Snellman) and U.S. Pat. No. 3,955,855 (Massieon et al). In many types of terrain these vehicles provide improved performance relative to the performance of pneumatic tired vehicles; still, difficulties are encountered with existing endless track vehicles.
Originally, such tracks were made of a plurality of metal links or shoes pivotally attached to each other to form an endless track which are very heavy, cause serious damage to roads and other surfaces on which they run and result in an uncomfortable ride for the passengers. See for example U.S. Pat. No. 2,823,082 (Bauer) and U.S. Pat. No. 4,530,546 (Meisel, Jr.). Heavier vehicles still use metal tracks.
The above referenced conventional tracks have the disadvantages of being noisy and vibration prone, not sufficiently durable and/or not usable on road surfaces. This is because of the conventional configurations and use of metal parts. Accordingly, substantial efforts have been made to construct quieter, smoother operating and more durable tracks for tracked vehicles.
A need has developed for a form of vehicle appropriate for both normal highway use and off road use over snow covered, very uneven, or muddy terrain. There is significant need for such a vehicle not only during armed conflicts but also following natural emergencies (snow and wind storms, floods, etc.), and such vehicles are at present particularly needed in developing countries. Unfortunately, almost all available automotive vehicles require infrastructure (paved highways, bridges, etc.) for practical operation, and the developing countries are decades away from having the necessary infrastructure for such conventional vehicles. Further, most load carrying off road vehicles presently in use have either very large wheels or very cumbersome tracks which are heavy, slow moving, and inappropriate for use on paved roads at normal highway speeds.
Rubber endless tracks have become popular due to an increase in construction in urban areas where vehicles having tracks must drive on the pavement and because there is a demand for low soil compaction farming. With the combination of rubber technology and a tremendous amount of trial and error, various types of rubber tracks are now available in the industry. They are used on excavators, dump carriers, boring machines, combines, tractors, and the like. See for example U.S. Pat. No. 5,279,378 (Graiwey & al) and U.S. Pat. No. 6,267,458 (Hansen et al). Most of them operate on the job site only and are transported between sites by trucks or trailers. To eliminate the inconvenience of transporting the vehicle between job sites, a vehicle which can operate both on public roads at normal speeds and in off road construction environments is required. However, such a vehicle will have to be constructed so as to provide little damage to the pavement, include less vibration and noise, and operate with less maintenance and lubrication. Furthermore, vibrations to be transmitted to occupants are mitigated and paved roads are not significantly damaged.
A number of hybrid tracks have been proposed where the links or shoes are made of metal which is provided with a rubber cover or insert. See for example U.S. Pat. No. 2,359,586 (Sayler), U.S. Pat. No. 2,369,130 (Benson), U.S. Pat. No. 2,409,502 (Leguillon et al), U.S. Pat. No. 3,148,921 (Batur et al), U.S. Pat. No. 4,109,971 (Black et al), U.S. Pat. No. 4,359,248 (Kortering) and U.S. Pat. No. 4,588,233 (DenBesten).
3. Description of the Related Art
[NOTE: As used herein, the term “rubber” relates to any elastic and primarily non metallic materials such as rubber, elastomers, or combinations thereof used in the manufacture of endless tracks].
Most rubber tracks are formed around a basic carcass or belt. The carcass includes an endless belt shaped rubber like elastic member, a number of core bars (usually of metal) embedded therein and aligned in the longitudinal direction thereof and extending in traverse directions thereof, and steel cords (tension resistant members) embedded in the endless elastic member to surround the core bars circumferentially outwardly. See for example U.S. Pat. No. 4,904,030 (Ono), U.S. Pat. No. 5,295,741 (Togashi et al), U.S. Pat. No. 5,511,869 (Edwards et al) and U.S. Pat. No. 6,241,327 (Gleasman).
Some have suggested the construction of endless rubber tracks using a plurality of interconnected polymeric modules. See for example U.S. Pat. No. 4,861,120 (Edwards et al) U.S. Pat. No. 5,005,922 (Edwards et al).
Terrain contacting lugs are formed integral with the exterior surface of this basic belt element. Known rubber tracks include large lugs having a variety of well known orientations, e.g., formed generally perpendicular to the track axis, or at an angle to the track axis, or in a chevron or modified chevron design. These latter special tracks also include interior lugs or horns for maintaining the track in alignment as it travels over the circumferences of the rubber tired wheels, such lugs being located either in the centre of the interior surface of the track (for designs appropriate for fitting between the tires of dual wheels) or in two aligned rows near the outside edges of the track (for receiving a single tire there between). See for example U.S. Pat. No. 5,447,365 (Muramatsu & al) and U.S. Pat. No. 5,540,489 (Muramatsu & al).
The tracks are carried by a plurality of rotating elements (wheels, sprockets, etc. . . . ) mounted on the track laying vehicle, the tracks being maintained in circumferential contact with these rotating elements and being driven thereby (or, in the case of trailer-like non driven vehicles, being supported for rotation thereon).
Problems encountered in actually reducing such an endless rubber track to practice include how to maintain adequate tension on such belt, how to drive such tracks and keeping the belt in lateral alignment with the wheels when the wheels are subject to large lateral loads. Other problems are maintaining the structural integrity and providing long life for the belt, mid rollers, drive wheels, and idler wheels.
While smaller rubber tracked vehicles are commercially available, these do not carry adequate loads for military vehicles (including tanks), normal multi passenger or produce transports, and their drive wheels can easily become mired in heavy mud or snow.
As stated in U.S. Pat. No. 5,295,741 (Togashi et al), when a vehicle equipped with rubber tracks moves on sandy terrain or quarries, the rubber tracks are likely to shift from advancing directions of the vehicle due to elongations and contractions of the rubber-like material in vertical and horizontal and other directions. As a result, the rubber tracks unavoidably get off the sprocket wheels or track rollers of the vehicle. Various attempts have been made in order to prevent the dislodgement of the rubber tracks from the vehicle, but they have not met with success.
Known rubber tracks, when mounted on the rotating wheels of vehicles, exert distinct resistive forces that must be overcome to move the vehicle, i.e., resistive forces in addition to those forces created by the load being carried and/or generated by the terrain. These further resistive forces relate to the additional tensions required to stretch the heavy lugs of the tracks around the wheels over which they are mounted and to the additional friction generated between the tracks and the terrain. While the latter frictional resistive forces are a valuable attribute under wet or snowy conditions, they add undesirably to energy costs when driving the vehicle over flat, hard surfaces.
The aforementioned patents are representative of a large body of patents which purport to solve one or more of the rubber track system implementation problems. Such body of patents constitutes documentary evidence that efforts to achieve this blend of track and wheel propulsion systems have been exerted for over half a century without realizing any practical measure of success. Solutions to the problems of actually implementing a heavy-duty vehicular rubber track drive system have proven elusive and scientific scaling techniques have not, to date, been successfully applied to light duty vehicles for purposes of developing a heavy duty rubber track system. Thus, despite the long felt need for and the advantages thereof, a heavy duty application vehicle utilizing such rubber track system is commercially unavailable today.
For this reason, most military tracked vehicles are still equipped with metallic tracks. In an effort to reduce the inconveniences related to such metallic track systems, some tracks are provided with rubber pads on the ground engaging side of the metallic track.
However, these metallic tracks still present significant inconveniences in relation to wheeled vehicle. Some of these problems can be summarized as follows:                Noise. The metallic track produces an excessively high level of noise. This fact can cause a significant strategic disadvantage when used in association with military vehicles since the enemy can detect the presence of the military vehicle many miles away.        Damage. With respect to civilian vehicles equipped with such tracks or military vehicles used in peace keeping missions, the metallic tracks can cause significant damage to the ground surface whether it be paved or not.        Weight. Metallic tracks are very heavy. For example the typical weight of a metallic track used on an M113 tank is 1200 pounds while the metallic track used on a Bradley tank is 2500 pounds. Such a weight is both an inconvenience with respect to the mobility of the vehicles and with respect to their consumption of fuel.        Short lifespan. Metallic tracks have a short lifespan. Even metallic tracks provided with rubber pads wear out extremely rapidly such that the pads need to be replaced every 500 to 1000 miles on a military tank.        Maintenance. Metallic tracks also require a lot of maintenance. The replacement of the rubber pads, the metallic links or shoes, etc. requires a continuous maintenance of the tracks.        Costs. Finally, the cost of manufacturing, maintenance and refurbishing are extremely high.        
As seen above, efforts to develop a rubber alternative to metallic tracks have been relatively successful in relation to “light” vehicles. However, such rubber tracks, although an improvement in many respects over the known metallic tracks still have the following shortcomings in relation to heavy equipment such as tanks:                De-tracking. De-tracking is a phenomenon by which a track loses contact with the guiding system such that it will completely remove itself from the vehicle and thus cause it to stop. To limit this phenomenon, more or less rigid protuberances or guide horns are disposed along the interior portion of the track to form a rampart which fits between guide wheels. These wheels are generally grouped in pairs and the space between the wheels allows the guide horns to past freely between them. Another element which is important to reduce de-tracking is the use of an appropriate tension wheel.        Teeth Skipping. Tracks are powered by a motor mechanically connected to a sprocket which engages protuberances or drive lugs on the inside surface of the track. If these drive lugs are allowed to skip over the teeth of the sprocket, damages to and premature wear of the track will occur. The abrupt movements which result also cause significant discomfort to the passengers of the vehicle. In order to diminish such teeth skipping phenomenon, the drive lugs on the track must produce as small a friction as possible on the sprocket while allowing the sprocket to firmly engage the track. Again, tensioning of the track is a critical element.        Wear and tear. Tracks can also prematurely break down due to a number of other internal and external elements such as: breakage of reinforcing rods, tearing of the reinforcing fabric, cables and fibers and the abrasion, piercing, tearing and de-lamination of the rubber components.        
The prior art solutions which are adapted for certain industrial and agricultural vehicles are inadequate when faced with the severity of the problems faced by military vehicles. Indeed, the high weight of military vehicles combined to the high speeds and strong accelerations (both lateral and longitudinal) create high stresses in the track which considerably amplify the aforesaid problems.
It is to be understood that the weight of certain tracked vehicles such as military vehicles, and more particularly, the weight of tanks, can be relatively high. For example, a M1 battle tank can weight approximately 55000 kilograms, a M2 Bradley tank can weight around 23000 kilograms and a M113 transport vehicle can weight around 10000 kilograms. Thus, as used herein, the term “heavy vehicle” relates to any track laying vehicle weighing at least 6000 kilograms whereas the term “light vehicle” relates to any track laying vehicle weighing less than 6000 kilograms.